1. Field of the Invention
The present invention relates to a fixing structure for a strain gauge type sensing element, which is available for measuring a loading weight of a vehicle, e.g., truck.
2. Description of the Related Art
Recently, a vehicle loaded with heavy luggage, e.g., truck, suffers from troubles. For example, it causes traffic accident, e.g., its turning over on its side during its running, and quickens the deterioration of the vehicle per se or the road. One of the causes of such troubles is overloading. A measure having been taken to prevent this overloading is to measure a loading weight of the vehicle, i.e., a load acting on the vehicle.
By convention, the vehicle load is measured in a state that the vehicle to be measured is placed on a platform balance. Accordingly, the measuring equipment is large and a large space is required for its installation. For this reason, the number of platform balances that maybe installed is limited, and their installation cost is high.
Recently, a load measuring apparatus is used which measures a vehicle load in a state that the apparatus is mounted on the vehicle per se.
A shackle is interposed between a first end of a leaf spring which is coupled at a second end to a load carrying platform through a bifurcated bracket and another bracket coupled to the load carrying platform. In a load measuring apparatus disclosed in JP-A-8-313332, based on this structure, a strain gauge type sensing element for measuring a vehicle load, such as a strain gauge sensor, is attached to within a shackle bin, which is used for coupling the shackle to the bracket in a swingable manner. A load is computed based on a total of measured values derived from a plurality of sensing elements corresponding to the wheels.
Each load sensor is located at the central part of a plate member covered with an insulating film. The load sensor includes a resistor portion in which four resistors, which intersect the plate member at an angle of 45° in both the longitudinal and width directions, are connected in a substantially rectangular form, and four terminals located at the connecting points of the resistors.
When one of the bracket and the shackle is moved to the other by a load acting on the vehicle, a shear force acts on the plate member in the width direction, so that resistance values of the resistors vary and a magnitude of the load is detected.
In the load measuring apparatus of JP-A-8-313332, the sensing elements are inserted into the containing parts of the shackle and fixed there. Accordingly, the sensing element sometimes is loosened and slightly moves. This makes the measurement accuracy poor.
Further, in the convention load measuring apparatus, it is necessary to bore the shackle to insert the sensing element into the bored location, and to lead a lead wire out of the shackle bin. Accordingly, the assembling work to the sensing elements to the load measuring locations is troublesome, and the shackle bin used for the load measuring location is weakened in strength.
Accordingly, an object of the present invention is to provide a means to solve the defects of the conventional load measuring apparatus.
To achieve the above object, there is provided a fixing structure for a sensing element, where in a load sensor is located at the central part of a plate member as a support member of a sensing element in the longitudinal direction, the plate member is attached to a load measuring location so that a length of the plate member when viewed in the longitudinal direction expands and contracts in accordance with a tension force or a compression force applied to the load measuring location, and both ends of the plate member when viewed in the longitudinal direction are welded to the load measuring location.
In the fixing structure, the plate member is welded to the load measuring location. Accordingly, the sensing element may easily be attached to the load measuring location. Accordingly, the fixing structure prevents the plate member from being loosened and slightly movable, and hence ensures an accurate measurement of the load. If the measurement is repeated, the measurement accuracy is not degraded. The sensing element is welded at only two positions. When comparing with the fastening at three points or larger, generation of a welding stress in a torsional direction in the sensing element maybe prevented, and hence a stable strain detection is secured. Since the welding is made at the locations spaced furthest from the load sensor, the load sensor is less affected by heat conduction.
In the fixing structure for a sensing element, the plate member is so thin as not to be buckled by a compression force applied to the load measuring location.
Since the plate member is thin, appropriate welding to the load measuring location is secured. Since the plate member is thin to such an extent as to avoid deflection by the buckling, an accurate load sensing is ensured.
In the fixing structure for a sensing element, the load sensor is arranged such that four resistors made of conductive material are bridge connected on an insulating area formed on the reverse side of the plate member, the resistors are connected at four connection points, and voltage is applied to the two connection points located on a diagonal line, and a potential difference appears between the two connection points located on the diagonal line and varies in accordance with a compression force or a tension force acting on the plate member, and the two resistors of the four resistors are oppositely disposed on the insulating area in the longitudinal direction of the plate member, while the remaining paired resistors are oppositely disposed on the insulating area in the widthwise direction.
In the fixing structure thus arranged, a compression force or a tension force applied to the plate member through the welding parts acts to the resistors to vary resistance values of them. A load acting on the load measuring location may be correctly detected.